Q.The stoichiometric defect that decreases the density of an ionic solid is ................. .

A Frenkel defect only relocates an ion internally (from a regular site to an interstitial site), so no mass leaves the crystal and its density is unaffected — the density decrease and the 'ions leaving the crystal' description in this question both actually describe a Schottky defect, not Frenkel.

Checking the assertion: in a Frenkel defect, a smaller ion (typically the cation, due to its smaller size) leaves its regular lattice site and occupies a nearby interstitial void, while remaining within the crystal. Since no ions actually leave the crystal (mass is conserved, just relocated internally), the overall density of the crystal is unchanged by a Frenkel defect. This is precisely what distinguishes it from a Schottky defect, where equal numbers of cations and anions genuinely leave the crystal, creating vacancies and decreasing the density. So the assertion — that Frenkel defect decreases density — is false.

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Schottky defect (equal numbers of cation and anion vacancies) occurs typically in ionic solids like NaCl where the two ions are similar in size.

A Schottky defect is a point defect in ionic crystals in which equal numbers of cations and anions are missing from their normal lattice sites (vacancy defect), leaving the crystal electrically neutral overall. It lowers the density of the crystal and is favoured when the ionic sizes of cation and anion are not very different, and coordination …

Frenkel defect (a smaller ion, usually the cation, dislodges to an interstitial site) is favoured when there is a large size difference between the two ions, as in silver halides.

A Frenkel defect occurs when an ion (usually the smaller cation) leaves its regular lattice site and occupies an interstitial site, creating a vacancy at its original position — overall composition and electrical neutrality are unchanged, but density stays essentially the same.

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A Schottky defect is a point defect where equal numbers of cations and anions are missing from their lattice sites, keeping the crystal electrically neutral; it occurs in ionic solids where the cation and anion are of comparable (similar) size and have a high coordination number — NaCl is the classic example.

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A Schottky defect removes equal numbers of cations and anions from the lattice, so mass is lost while the crystal's overall volume stays essentially the same — this lowers the density.

A stoichiometric defect keeps the cation : anion ratio (the formula) of the ionic solid unchanged.

• Frenkel defect — a smaller ion (usually the cation) leaves its normal lattice site and sits in an interstitial site instead. No particle actually leaves the crystal, so both mass and volume stay the same and density is unaffected (e.g. AgCl, AgBr, ZnS). …